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Exposure Factors
| Question | Answer |
|---|---|
| More mAs | No effect on quality |
| Increase mAs | increase quantity |
| Increase kVp | increase x-ray quality |
| Increase SID | No effect on quality |
| Increase Filtration | Increase Quality of x-ray beam |
| Increase mAs | Increase quantity |
| Increase SID | Decrease quantity reaching film |
| Increase kVp | Increase quantity |
| Increase filtration, decrease quantity | Increase quality |
| Increase kVp | Decrease contrast |
| Attenuation | When x-ray beam is reduced as it travels through matter due to patient absorption and scattering |
| Quality of x-rays measured by HVL | A higher HVL means freater quality x-ray beam |
| HVL | is the thickness of absorbing material needed to reduce the x-ray intensity to half its orginal value |
| Inherent | about .5 mm AL |
| Added filtration | 2mm-a thin sheet of aluminum is added between tube housing and collimator |
| Density | mAs |
| Contrast | Difference between light and dark |
| Change the SID | Density changes |
| Total minimal beam filtration | 2.5 mm Al |
| kVp affects | Contrast and density |
| Go down in kVp (x by .85) | Double the mAs |
| Go up in kVp (x by 1.15) | 1/2 the mAs |
| Bone | Radiopaque-lighter |
| Tissue | Radiolucent-darker |
| Emulsion layer | Active layer-Silver halide crystals-contain sensitivity centers |
| Sensitivity centers | where the latent image is formed |
| Speed | how fast does film respond to x-ray and light |
| Fast screen | film needs fewer x-rays to produce an image |
| Contrast | Ability to visualize gray shades |
| High contrast film produce | black and white images |
| Low contrast film produce | shades of gray |
| Resolution | Ability to detect details-sharpness |
| Latent | before chemicals-the visible image after exposure and before chemical processing |
| Visible | After development-the image evident after chemical processing |
| Artifacts | Undesired mark or images that appear on radiographs |
| Developing | 1st stage-latent image is converted to a visible image |
| Fixing | Clears the unexposed silver halide crystals from the film emulstion and hardens the film emulsion to preserve the image |
| Washing | Removes the excess chemicals |
| Drying | Removes the water and prepares radiograph for viewing |
| Daylight processing | No darkroom needed-cassettes where fed into processor and then film was removed |
| Intensifying Screens | Amplify (intensifies) the effect of x-rays srtiking the screen |
| Phosphor in screens | concerts x-ray beam into light |
| Screen speed | The faster the speed the less radiation needed |
| Luminescence | Any material that emits light in response to outside stimulation such as phosphorus |
| Fluorescence | Visible light continues to be emitted after stimulation stops-Screen lag or afterglow-not good |
| mAs1/mAs2=screen speed factor2/screen speed factor1 | formula for screen speed |
| Spatial Resolution | The ability of an intesifying screen to produce an accurate and clear image |
| Spatial Resolution measurement | line pairs per millimeter (lp/mm) |
| High Speed screens have | low spatial resolution |
| Low speed screens have | high spatial resolution |
| Image noise | Quantum Mottle |
| Quantum Mottle | Appears as speckled background on image |
| Fast screens | high kVp and too low mA-Quantum Mottle |
| Screens and films are | designed to match |
| Wire mesh test | to check for a uniform exposure across image |
| Long Scale | Slight differences between densities-more grays but more densities-high kVp-less contrast-abdoment x-ray |
| Short Scale | Major differences-blacks and whites-total # of densities reduced-lower kVp-more contrast-bones |
| When x-rays interact with matter the do so in | Compton Effect or Photoelectric Effect |
| Compton Effect/Scattering | The incident (incoming) x-ray interactions with and outer-shell electron* and ejects it from atom-results in release of scatter x-ray |
| Photoelectric Effec/Absorption | The incident (incoming) x-ray is totally absorbed by an inner-shell electrom* |
| Comton effect-As kVp increases | you have more scattered radiation and more x-rays are trasmitted through the patient |
| Atomic number (Z#) of the absorber (body part) doesn't effect | Compton scattering |
| As the mass density of absorber increases so do the amount of | Compton scattering-thicker patients-more scatter |
| Scatter radiation from Compton | Reduces image contrast-BAD!! |
| Higher kVp is better for the patient but | more scatter |
| Photoelectric Effect (PE) | Absorption!! |
| As kVp increases there is less PE compared to | Compton |
| As the atomic number (Z#) increases so does the amount of PE (directly proportionalal to Z cubed), therefore | more absorption in bone than muscle |
| Higher kVp (energy, quality, speed electrons, penetrability) | More Compton, but overall more transmitted through patient |
| What is a GRID | A device used to reduce the level of scatter radiation that reaches the IR |
| What is identified by different ratios? | Grids-5:1, 8:1, 12:1, or 16:1-higher the grid ratio the more scatter reduction |
| Grids improve | image contrast** |
| Grids don't reduce... | patient dose |
| If body part is thicker than 4-5 inches | use grid |
| As the grid ratio increases so does... | the amount of mAs needed-direct relationship |
| grid/density equation | mAs1/mAs2=grid factor1/grid factor2 |
| Generally the factors that increase the intensification factor (IF) reduce spatial resolution so therefore... | high speed screens have low spatial resolution |
| Scatter radiation | not good-causes reduced image contrast-increases patient dose-increased image density |
| As kVp increases so does the amount of | Compton Scattering |
| As kVp increases the amount that goes through the patient is | increased |
| Only 1% of x-ray incedent on patient... | reach the IR |
| Field size-collimation | Greater field size results in more scatter radiation and less image contrast |
| You will have to increase your ______________ to compensate foe increased collimation | exposure factors |
| Compression | Improved overall spatial resolution (detail accuracy) |
| Compression improves... | Contrast resolution (ability to detect differences in tissue**) |
| Compression lowers... | patient dose-thickness decrease (essential in mammography) |
| Devices to restrict beam | Aperture diaphragm, cones and cylinders, variable aperture collimator |
| Aperture diaphragm | Lead-lined metal diaphragm |
| Cones and cylinders | Extensions from collimator head |
| Variable Aperture Collimator | Shuters that limit size of beam-what we use |
| PBL-Position beam limiting | Required by law and a type of automatic collimation |
| should your light beam size be greater than the IR size? | NEVER!! |
| Grid strips made of... | lead |
| Interspace material | Aluminum or plastic fiber and must be nonhydroscopic (doesn't absorb moisture) |
| Grid ratio equation | H/D where H is the height of the grid and D is the width of the interspace material |
| Higher grid ratio... | better in clean up but more patient dose |
| Grid frequency | The number of grid strips per centimeter |
| Contrast improvement factor | K |
| K equation | pt dose with grid (patient gets more radiation)/pt dose without grid |
| As bucky factor increases so does... | Technique required and patient dose |
| Types of grids... | Parallel, Crosses, Focused |
| parallel grids | linear-all lead strips are parallel |
| Grid "cut-off" | Undesirable absorption of primary x-rays by the grid-most common in parallel grids |
| papallel grids use... | higher technique and must be perpendicular |
| Cross Grids | Grid strips run paralle to both axes of grid-cleans up more scatter than parallel |
| Focused Grids | Grid strips are designed to be angled so they are parallel to diverging x-ray beam-matches beam coming out of tube |
| High ratio grids have.... | less positioning latitude (room for error) than low-ratio grids |
| Moving grids | if the grid in the table didn't move you would see grid lines |
| grid moves no more than ________ dring exposure | 2-3 cm (about 1 inch) |
| Grid problems... | Off level, off center, off focus, upside down, or off-centered and off focus |
| What ratio grid to use | Generally above 90 kVp use grid ratios above 8:1 |
| Air gap | Reduces scatter but causes magnification (OID-increased SID) of image. ex-lateral C-spines |
| The air does or doesn't filter out x-rays? | DOES NOT!! |
| Manifest image | Can see |
| Latitude | Range of exposures to get acceptable images-room for error |
| Faster speed... | Less x-ray (exposure) needed-more sensitive-higher Z# |
| Greater film speed means... | less image detail/resolution |
| DQE | Detective Quantum Efficiency |
| Detective Quantum Efficiency (DQE) | ability of phosphor to absorb x-rays, so a high Z# of phosphor is desired |
| CE | Conversion Efficiency |
| Conversion Efficiency (CE) | Phosphor needs to emit large amount of light per x-ray absorption |
| Latent image | The invisible change that is induced in the silver halide crystals. |
| latitude | the range of exposure techniques that produce an acceptable image. |
| The latent image is | Formed in the film emulsion when light photons interact with the silver halide crystals |
| Speed | The sensitivity to the screen-film combination to x-rays and light. |
| Fast screen-film | Need fewer x-rays to produce a image |
| Spectral matching | Intensifying screens emit light when exposed to x-rays and the emitted light then exposes the film |
| Daylight processing | The film is automatically extracted from the cassette and is sent to the processor. |
| Intensifying screens | Amplifies the effect of image-forming x-rays that reach the screen-film cassette. Lowers pateint dose considerably. |
| Phosphor | Converts the x-ray beam into light |
| The Active Layer | Phosphor |
| Detective Quantum Efficiency | The phosphor should have a high atomic number so that x-ray absorption is high. |
| Conversion efficiency | The phosphor should emit a large amount of light per x-ray absorption |
| Spectral matching | The light emitted must be of proper wavelength (color) to match the sensitivity of the x-ray film. |
| Phosphor afterglow | The continuing emission of light after exposure of the phosphor to x-rays, should be minimal. |
| Luminescence | Any material that emits light in response to some outside stimulation |
| Outer-shell electrons | Luminescence occurs when an outer-shell electron is raised to an excited state and returns to its normal state with the emission of a light photon. |
| 2 types of luminescence | Fluorescence and phosphorescence |
| Fluorecence | If visible light is emitted only while the phosphor is stimulated |
| Phosphorescence | If the phosphor continues to emit light after stimulation |
| Screen lag or afterglow | Phosphorescence in an intensifying screen-dot desirable |
| intensifying screen characteristics | Screen speed, image noise, and spatial resolution |
| Intensification factor | is a measureof the speed of the screen |
| Image noise | With an increase in screen speed can result in increased speckled appearance on some images. |
| Spatial resolution | The ability to produce a accurate and clear image-when image-forming x-rays are converted to visible light and the visible light in turn produces the latent image, the image is blurred somewhat. |
| Screen speed | A relative number the describes how efficiently x-rays are converted into light-HAS NOTHING TO DO WIT PATIENT DOSE! |
| Image noise occurs when | fast screens and high kVp techniques are used. Noise reduces image contrast |
| The amount of light emitted for each x-ray absorbed is higher... | Higher conversion efficiency (CE) results in increased noise. |
| Image noise increases with... | higher conversion efficeiency (CE) but not with higher detective quantum efficiency (DQE). |
| Quantum Mottle | Often is a direct result of use of very fast speed screen-film-result in a grainy, mottled, or splotchy image. |
| Spatial resolution refers to | how small an oblject can be imaged |
| Contrast resolution refers to | The ability to image similar tissues, such as liver and pancreas or gray matter and white matter |
| Image detail | spatial resolution and contrast resolution |
| Line pairs per millimeter | lp/mm-the higher this number the smaller is the object that can be imaged and the better spatial resolution |
| Reduced spatial resolution | increased image blur |
| High-speed screens... | have low spatial resolution |
| fine-detailed screens | have high spatial resolution |
| Intesifying screens | Change the energy og the image-forming x-ray beam exiting the patient into visible light, which exposes the radiographic film. |
| Scatter radiation produced by the Compton effect produces | Noise, reducing image contrast and contrast resolution-it makes the image less visible |
| What 3 factors contribute to increased scatter radiation | Increased kVp, increased x-ray fild sixe, and increased patient thickness. |
| Beam restricted devices | Are designed to control and minimize scatter radiation by limiting the x-ray field sixe to ony the anatomy of interest. |
| Grids remove... | a major source of noise, thus improving image contrast |
| 2 principle tools are used to control scatter radiation | beam-restricting devices and grids. |
| Density | Amount of blackness |
| What should you do if an image is underexposed? | Double mAs |
| Short scale contrast results in high or low kVp? | Low |
| Best way to reduce scatter | Collimate |
| how to compensate for involuntary contraction-peristalsis | decrease time |
| table bucky vs table top | table bucky has increased magnification vs table top |
| Radiowaves, light and x-rays are examples of | electromagnetic (energy of x-rays) |
| Ionization | removal of electron |
| As kVp increases | PE absorption decreases |
| the 2 basic components of film | base and emulsion |
| Intesification Factor increases when... | kVp increases |
| Ability of the intesifying screen to absorb x-rays | CE-Conversion efficiency |
| Decreasing kVp will... | Increase patient dose |
| PE interactions increase when... | kVp decreases |
| Compression device will increase or decrease contrast? | Increase-better-less thickness |
| Filtration | Increases the quality of beam (kVp) by 10%, decreases exposure by 1/2, and decreases the long wavelengths in the beam. |
| Reducing the number of low energy photons reaching the patient | Filtration |
| compensating filter | A more uniforn optical density of structures of unequal thickness can be accomplished |
| Speed of the photons is Never altered by... | filtration |
| Higher kVp and filtration will achieve... | higher quality |
| Optical density | Amount of darkening |
| Primary function of mAs during an exposure is | the regulation of optical density in the image |
| High optical density | air and gas-lower atomic number-darker |
| Barium should use what kVp range | 90-110 kVp |
| Excessive optical density | Darker than normal |
| An increase in 15% kVp will result in a... | decrease in mAs by 50% to maintain the same optical density |
| The dose a patient recieves is directly proportional to the | mAs employed |
| Insuffiecient contrast and excessive optical density | Decrease mAs and kVp |
| Low kVp will have the lowerst absorption | free air or gas-finger or hands |
| Contrast | Differences in he optical densities between adjacent areas of the radiograph |
| Soft beam | low quality beam |
| To see a noticable change in optical density... | a change of 30% in the mAs is required |
| An increase in how much kVp should be used for every centimeter of thickness difference? | 2 kVp |
| Any material that greatly reduces the number of incident x-ray photons passing through the material is... | Radiopaque |
| Tissue with a low tissue density will normally have | high optical density or appear dark |
| The thickness of the tissue will have an | increase in scatter produced by structure |
| When an air gap technique is used, the amount of scatter reaching the IR is decreased because... | more scatter misses the IR |
Created by:
lkunze